Microstrip patch antennas have several well known advantages over other antenna structures. These antennas generally have a low profile and conformal nature, are lightweight, have low production cost, are robust in nature and compatible with microwave monolithic integrated circuits (MMICs) and optoelectronic integrated circuits (OEICs) technologies. However, one drawback of such devices is their relatively narrow bandwidth. In order to achieve wider bandwidth, a relatively thick substrate must be used. However, the antenna substrate supports tightly bound surface wave modes which represent a loss mechanism in the antenna. The loss due to surface wave modes increases as the substrate thickness is increased. It is desirable to develop conformal microstip antennas which enjoy wide bandwidth, yet do not suffer from the loss of attractive features of the conventional microstrip patch antenna.
One way to enhance the mutual coupling antenna element-to-antenna element performance is to surround the patch elements with metal walls. This technique effectively prevents surface wave modes from being excited in a substrate, thus allowing the substrate's thickness to be increased without serious effects. In addition to the common techniques of increasing patch height and decreasing substrate permittivity, a conventional method uses parasitic patches in another layer (stacked geometry). However, this has the disadvantage of increasing the thickness of the antenna. Parasitic patches can also be used in the same layer (coplanar geometry); however, this undesirably increases the lateral size of the antenna and is not suitable for antenna array applications.
In many applications, such as phased array radars, low profile antennas are required; therefore, microstrip antennas are often utilized. The microstrip antenna is constructed on a thin dielectric sheet using printed circuit board and etching techniques. Three common geometries, rectangular, square and round, are widely employed. Circular polarized radiation can be obtained by exciting the square or round element at two feed points 90° (degrees) apart and in quadrature phase. A direct probe connected patch antenna element which is suitable for application at low UHF frequencies is required for a phased array application. The impedance matching of such an antenna should be compact, mechanically simple, and take advantage of the volume occupied by the patch antenna element. A broad band antenna element requires the use of thick substrates with low relative dielectric constants approaching that of air.
As indicated above, patch antenna elements are employed in phased array radars and other phased array situations where low profile antenna elements are required. A patch antenna array often is constructed as a single printed circuit board with ground plane on one side and patches on the second radiating side. See for example, the above-noted patent application entitled, “Tuning Apparatus For A Probe Fed Patch Antenna”, filed on Mar. 5, 2007 having Ser. No. 11/713,914. That application describes microstrip patch antennas and more particularly a tuning apparatus for such an antenna. The application also shows the above-noted patch antenna array configuration. In any event, in a patch antenna array the individual patches are often open laterally. The laterally open substrate can support surface waves and mutual coupling between adjacent antenna elements is strong. A method to reduce mutual coupling is to mount individual patch elements in metallic cavities. The cavity mount structure prevents propagation of surface waves in substrates since the substrate's size is limited to the immediate area around each patch.
In a phased antenna array, each antenna element may be required to be individually removable. The patch antenna element typically is required to fit a physical lateral envelope of about 0.5 by 0.5 wavelengths and also to accommodate a specified total thickness. A metal wall cavity structure is able to reduce the antenna element to antenna element mutual coupling. The presence of metal cavity sidewalls near a stacked patch configuration increases the coupling co-efficient between the stacked patches.
As one will understand, the present invention discloses the use of septa or partitions to control coupling coefficients from patch to patch. The use of septa allows for reduction of total cavity thickness and enables improved bandwidth control.